Neural tissue can be artificially stimulated and activated by prosthetic devices that pass pulses of electrical current through electrodes on the prosthetic devices. The passage of current causes changes in electrical potentials across visual neuronal membranes, which can initiate visual neuron action potentials. These visual neuron action potentials are the means of information transfer in the nervous system.
Based on this mechanism, it is possible to input information into the nervous system by coding sensory information as a sequence of electrical pulses relayed to the nervous system via a prosthetic device. In this way, it is possible to provide artificial sensations including vision.
One typical application of neural tissue stimulation is in rehabilitation of the blind. Some forms of blindness involve selective loss of light sensitive transducers of the retina. Other retinal neurons remain viable, however, and may be activated in the manner described above by placement of a prosthetic electrode device on the inner (toward the vitreous) retinal surface (epiretinal). This placement should be mechanically stable, minimize distance between the prosthetic device electrodes and the visual neurons, control electronic field distribution and avoid undue compression of the visual neurons.
Each person's response to neural stimulation differs. In the case of retinal stimulation, even a single person's response may vary from one region of the retina to another. In general, the retina is more sensitive closer to the fovea. Also worth noting for neural stimulation is that stimulation less than a minimum threshold value would be ineffective in eliciting perception. On the other hand, stimulation beyond a maximum level would be painful and possibly dangerous to a patient. It is therefore important to map any video image to a stimulation range between a minimum and a maximum for each individual electrode. With a simple retinal prosthesis with only one or very few electrodes, it is possible to adjust the stimulation manually by stimulating and questioning the patient.
The human retina includes about four million individual photoreceptors. An effective visual prosthesis may include thousands of electrodes or more. As resolution and number of electrodes increase, it may become difficult to adjust each electrode separately by stimulating and eliciting a patient response. Therefore, a system is needed to adjust the electrodes in a visual prosthesis with multiple electrodes for size, brightness and shape of percepts without need for patient interaction in a possibly long and difficult process of characterizing each electrode individually.